Phylogeny and evolutionary biology of Orcuttieae (Poaceae: Chloridoideae): Analysis of radiation into a unique amphibious environment.

摘要

Orcuttieae (Poaceae: Chloridoideae), an endangered tribe of vernal pool grasses, found only in California and Baja, California has been termed one of the most rare and unusual groups of grasses. Its uniqueness is defined by its rare habitat and the unusual photosynthetic attributes unexpected in aquatic grasses. This study includes analysis of photosynthetic adaptations, phylogenetic rooting, and phylogenetic analyses of Orcuttieae. The anatomical structure of aquatic, floating and terrestrial leaves of eight of the nine species of Orcuttieae as examined using light microscopy. Kranz anatomy was present in all floating and terrestrial leaves of Orcuttia californica, O. inaequalis, O. pilosa, O. tenuis, O. tenuis, Tuctoria greenei, T. mucronata, and Neostapfia colusana. Aquatic leaves of all members of Orcuttia lacked Kranz anatomy, while aquatic leaves of Tuctoria and Neostapfia possessed Kranz anatomy. Carbon isotope data from leaves at all stages were in the range -12.25 to -14.31, indicating that all species carry out C4, even with the absence of Kranz anatomy in the aquatic leaves of Orcuttia spp. These findings confirm and expand upon previous studies showing that some species lack Kranz anatomy while the other members do not. Inferring the root of Orcuttieae is difficult because its closest relatives have yet to be identified. A Bayesian approach with a molecular clock criterion was used to infer the root of Orcuttieae. The Bayesian technique gives significant evidence that the root of Orcuttieae is between the Neostapfia lineage and the Tuctoria/Orcuttia lineage. The third portion of the study is an analysis of morphology, chloroplast ( trnL intron) and nuclear ribosomal (ITS1&2) DNA sequence data to identify relationships within the tribe Orcuttieae. We conducted a phylogenetic analysis of published morphological, anatomical, physiological, and chromosomal data and our results are not consistent with relationships shown in the earlier studies. Maximum likelihood results of sequence data indicate Neostapfia and Orcuttia are both monophyletic, while Tuctoria is paraphyletic and intermediate between the previous two genera. Our data supports previously proposed hypotheses that Orcuttieae is derived from a terrestrial ancestor and is becoming more specialized to an aquatic environment.